Optoelectronic waveguiding device and optical modules

ABSTRACT

An optical transmission device suitable for a high-speed and large-capacity optical transmission system. An optoelectronic waveguiding device including an optical waveguide layer and cladding layers each having a larger band gap than that of the optical waveguide are deposited above and beneath the optical waveguide layer formed on a semiconductor substrate. The waveguide and cladding layers comprise optical waveguides each having a MQW structure in a direction of a light propagation axis of the optical waveguide layer. Among these optical waveguides, there exists first and second optical waveguides, whose layer structures may be mutually different. The optoelectronic waveguiding device maybe characterized in that an optical waveguide made of a bulk crystal exists in a connection part between the MQW structure waveguides each having a different layer structure. The specific connected optoelectronic waveguiding device elements may include semiconductor lasers, modulators and/or amplifiers.

PRIORITY TO FOREIGN APPLICATIONS

[0001] This application claims priority to Japanese Patent ApplicationNo. P2001-129178.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to integrated semiconductor opticaldevices, and more specifically, the present invention relates tosemiconductor optoelectronic waveguiding devices including an opticalwaveguide comprising a plurality of multiple quantum wells and tooptical modules utilizing such devices.

[0004] 2. Description of the Background

[0005] With the recent increase in the use of information andcommunication services, optical communication systems that support suchservices with higher speeds and increased capacity are desired. Forexample, optical transmission apparatuses for optical communication of atrunk line in which a plurality of communication lines are bundledshould support high-speed (10 Gbit/s class) and long-distancetransmission. Consequently, a semiconductor laser capable of operatingat a transmission speed as high as at least 10 Gbit/s is preferred asthe light source to be built into such optical transmission apparatuses.

[0006] A promising light source that enables high-speed, long-distancetransmission whose speed is equal to at least 10 Gbit/s is a laserdevice manufactured by integrating an EA (electro absorption) typeoptical modulator and a DFB (distributed feedback) semiconductor laseron a single substrate (“EA/DFB laser”). Because the operating principlesand availability of the EA/DFB laser are known, a detailed explanationis omitted and only the basic structure, advantages, and potentialdisadvantages thereof will now be described.

[0007] In the EA/DFB laser, areas of the DFB laser and of the EAmodulator are monolithically formed on the same semiconductor substrate.In general, a multiple quantum well (MQW) structure of the laser andthat of the modulator are formed to have different materials,compositions, layer thicknesses, and other properties so that an energygap of the MQW layer of the modulator part is larger than that of theMQW layer of the laser part. Typical methods whereby the MQW structureshaving mutually different energy gaps are formed on the samesemiconductor substrate include (1) a selective area growth method and(2) a butt-joint formation method.

[0008] Each of these methods has known advantages and disadvantages. Inorder to make full use of the EA/DFB laser in high-speed opticalcommunication systems (such as a transmission system whose transmissionspeed is equal to at least 10 Gbit/s), it is preferable to design boththe EA modulator and the DFB laser to have an optimal structureindependently so that each of these can exploit its own full potential.

[0009] In the case where the EA/DFB laser structure is formed by theselective area growth method, the modulator and the laser may be formedby single crystal growth; this method has the advantage of a simplemanufacturing process. However, the materials, the compositions, and thetotal numbers of layers of the laser part and of the modulator areinevitably the same, and the method has little or no room forindependent optimization of these device elements.

[0010] On the other hand, using the butt-joint formation method, thelaser part and the modulator can be formed by independent processes.Therefore, the modulator and the laser may be optimized independently interms of their materials, compositions, layer thicknesses, the numbersof layers, and other properties. For 10 Gbit/s-and-higher high-speed andlong-distance transmission, where both the modulator and the laser partpreferably each achieve high end characteristics respectively,integration of the modulator and the laser by the butt-joint methodappears promising.

SUMMARY OF THE INVENTION

[0011] When the butt-joint method is adopted, a laser having the opticalmodulator integral therewith is generally formed through the followingprocesses. (1) First, a device element structure of the DFB laser isformed on a semiconductor substrate. (2) The DFB laser area is protectedwith a dielectric film such as silicon oxide or silicon nitride. (3)Next, using the above-mentioned dielectric film as a mask, the MQW layerin the DFB laser area is selectively etched away to expose thesemiconductor substrate. (4) On the exposed semiconductor substrate, aMQW structure that is desired for use as the EA modulator and that willact as an absorption layer (“EA-MQW”) is re-grown.

[0012] In the growth process of the EA-MQW, because a feed rate ofgrowth species exceeds the normal feed rate (by a factor ranging fromzero to a few tens of μm) in the vicinity of the dielectric maskcovering the DFB laser area, the well layers and barrier layers of theMQW layer become thicker and the absorption edge of the MQW layer movestoward a longer wavelength. Additionally, it is known that the crystalquality of the EA-MQW layer in this area decreases. Details of thisphenomenon have been reported, for example, in the conference proceedingof IEEE lasers and electro-optics society, 9th annual meeting, WY2, p.189.

[0013] In the above-mentioned example, a crystal defect whose absorptionedge has moved to a longer wavelength and whose crystal quality hasdeteriorated easily absorbs light that propagates from the DFB laserpart to the EA modulator. Therefore, problems may be caused such as (a)a reduction in the optical output of the device and (b) the generationof unnecessary carriers in the EA modulator area. Similar problems werereferred to, for example, in Japanese Patent Application Laid-Open No.8335745. These potential problems are preferably addressed when thebutt-joint formation is used.

[0014] In at least one preferred embodiment, the present inventionprovides a semiconductor optoelectronic waveguiding device with as smallan optical loss as possible in the optical waveguide and that canrespond to high-speed modulation that is desired for high-speedtransmission. The present invention may also provide a semiconductoroptoelectronic waveguiding device that is manufactured by integratingdevice elements of the optical modulator and the semiconductor laser,each of which is optimized in terms of several characteristics thereof.

[0015] In at least preferred embodiment, the basic structure of thepresent invention comprises an optoelectronic waveguiding device havingat least two optoelectronic device element parts each comprising anoptical waveguide. The optical waveguides possessed by theabove-mentioned at least two optoelectronic device element parts arepreferably connected to each other with an optical waveguide such thatat least a core portion thereof is made of a bulk crystal. In this case,it may be convenient to form the above-mentioned at least twooptoelectronic device element parts on a single substrate, for example,a semiconductor substrate.

[0016] Examples of the above-mentioned at least two optoelectronicdevice element parts may include: a semiconductor laser part; an opticalmodulator part; and an optical amplifier part. Further, optical deviceelements other than these enumerated device elements may be used as theneed arises. Moreover, in accordance with particular embodiments of theoptoelectronic waveguiding device of the present invention, aconfiguration in which a plurality of pairs each having two of theseoptical device elements are arranged side by side may be adopted.

[0017] To achieve the maximum performance of the EA/DFB laser, thepresent invention preferably comprises a structure such that the crystaldefect between the DFB laser and the EA modulator is removed and awaveguide whose optical absorption is extremely low is inserted intothis area. Consequently, the basic structure according to at least oneembodiment of the present invention includes an optical waveguide of abulk crystal formed at a connection part between the MQW of the laserpart and the MQW of the modulator part. The form according to thepresent invention may be a structure whose quantum effect is extremelysmall by using the above-mentioned bulk crystal area as the opticalwaveguide.

[0018] The MQW of the above-mentioned laser part to be connected to theMQW of the above-mentioned modulator part is preferably formed by afirst butt-joint formation. The bulk crystal waveguide is then formed bya second butt-joint formation so as to have a desirable refractive indexdistribution.

[0019] It should be noted that it is preferable to establish an opticalconnection between the optical modulator and the laser part with theoptical waveguide in the present invention.

[0020] Thus, by adopting the optical waveguide made of a newly-formedbulk crystal, the present invention preferably realizes the one or moreof the following: (1) the optical absorption can be reduced as low aspossible by suppressing the shifting of the absorption edge that mayoccur in the MQW structure and (2) a further decrease in the opticalabsorption and improvement of the reliability of the device may beachieved by not providing a crystal defect of poor crystallinity in thedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] For the present invention to be clearly understood and readilypracticed, the present invention will be described in conjunction withthe following figures, wherein like reference characters designate thesame or similar elements, which figures are incorporated into andconstitute a part of the specification, wherein:

[0022]FIG. 1 is a perspective view of a device according to a firstembodiment of the present invention;

[0023]FIG. 2 is a cross section of a device according to the firstembodiment of the present invention, taken along a plane parallel to thetravelling direction of light therein;

[0024]FIG. 3 is a cross section of a device according to the firstembodiment of the present invention, taken along a plane intersectingthe travelling direction of light therein;

[0025]FIG. 4 shows cross sections of a device according to the firstembodiment of the present invention, illustrating a manufacturingprocess in a processing order;

[0026]FIG. 5 is a characteristic diagram illustrating the shift of theabsorption edge in a crystal defect of poor crystallinity;

[0027]FIG. 6 is a diagram in which cumulative distributions of slopeefficiency for the present invention and for the conventional exampleare shown for comparison;

[0028]FIG. 7 is a perspective view of a device according to a secondembodiment of the present invention;

[0029]FIG. 8 is a cross section of a device according to the secondembodiment of the present invention, taken along a plane parallel to thetravelling direction of light therein;

[0030]FIG. 9 is a cross section of a device according to the secondembodiment of the present invention, taken along a plane intersectingthe travelling direction of light therein;

[0031]FIG. 10 shows cross sections of a device according to the secondembodiment of the present invention, illustrating the manufacturingprocess in a processing order;

[0032]FIG. 11 is a cross section of a device according to a thirdembodiment of the present invention, taken along a plane parallel to thetravelling direction of light therein;

[0033]FIG. 12 is a view showing an example of a mask pattern to be usedto form a bulk crystal waveguide of a device according to the thirdembodiment of the present invention shown in FIG. 11;

[0034]FIG. 13 is a cross section of a device according to a fourthembodiment of the present invention, taken along a plane parallel to thetravelling direction of light therein;

[0035]FIG. 14 is a view showing an example of a mask pattern to be usedto form a bulk crystal waveguide of a device according to the fourthembodiment of the present invention shown in FIG. 13;

[0036]FIG. 15 is a top view of an embodiment that has an array of lightemitting device element parts;

[0037]FIG. 16 is a cross section of an embodiment that has an array ofthe light emitting device element parts, taken along a plane parallel tothe travelling direction of light therein;

[0038]FIG. 17 is a top view of an embodiment that has an array of lightemitting device element parts and an optical multiplexer;

[0039]FIG. 18 is a cross section of an embodiment that has an array oflight emitting device element parts and an optical multiplexer, takenalong a plane parallel to the travelling direction of light therein;

[0040]FIG. 19 is a top view showing an optical module that uses theoptoelectronic waveguiding device according to the present invention;

[0041]FIG. 20 illustrates the relation between radiation loss in thedevice and the distance between the laser part and the opticalmodulator;

[0042]FIG. 21 is a cross section showing a basic concept of the presentinvention; and

[0043]FIG. 22 is a cross section showing a comparative example where thesame single crystal is used to bury a gap between the laser part and theoptical modulator as well as the laser part and the modulator.

DETAILED DESCRIPTION OF THE INVENTION

[0044] It is to be understood that the figures and descriptions of thepresent invention have been simplified to illustrate elements that arerelevant for a clear understanding of the present invention, whileeliminating, for purposes of clarity, other elements that may be wellknown. Those of ordinary skill in the art will recognize that otherelements are desirable and/or required in order to implement the presentinvention. However, because such elements are well known in the art, andbecause they do not facilitate a better understanding of the presentinvention, a discussion of such elements is not provided herein. Thedetailed description will be provided hereinbelow with reference to theattached drawings.

[0045] Before the description of a concrete manner in which the presentinvention may be carried out, the effects that characterize the basicstructure of the invention and a comparison between the structure of theoptoelectronic waveguiding device according to the invention and that ofthe conventional optoelectronic waveguiding device will be furtherdescribed in detail.

[0046] In preferred embodiments of the present invention, theabove-mentioned crystal defect is removed, and hence, the absorptionloss of light in the device is significantly reduced. Because the bulkcrystal waveguide has no MQW structure, there may occur little shift ofthe bad-gap energy (absorption edge) in the vicinity of the mask patternedge in this butt-joint formation process. Therefore, the absorptionarising from the bulk crystal waveguide that is introduced into thestructure is decreased significantly as compared to that of the originalMQW structure.

[0047] By replacing the crystal in the crystal defect with a newly-growncrystal, the present invention may be differentiated from the methods inthe above-mentioned Japanese Patent Application Laid-Open No. 8-335745.Note that, in the above-mentioned Japanese Patent Application Laid-OpenNo. 8-335745, the absorption edge of the crystal defect is shiftedtoward a shorter wavelength by ion implantation to achieve reduction inthe absorption loss. In this case, although it is possible that theabsorption edge is shifted, the crystal defect itself cannot be removed.

[0048] Contrary to this, in the present invention, because the crystaldefect is preferably replaced with the optical waveguide of the bulkcrystal, not only can the absorption edge be shifted from the absorptionedge of the crystal having the crystal defect to a desired absorptionedge, but also the crystal defect itself may be removed. By the removalof the crystal defect in the optical waveguide, additional effects suchas an improvement in reliability of the device may also be expected.That is, the service life of an apparatus into which the present deviceis built may be improved. Further, according to the invention, it mayalso be possible to decrease the optical absorption in the opticalwaveguide as compared to the above-mentioned prior art.

[0049] Alternatively, if it is desired only to remove theabove-mentioned crystal defect, it is also possible that the crystaldefect is removed and subsequently the area is buried with the samematerial as the cladding layer.

[0050] Actually, a structure such that a gap between the DFB laser andthe EA modulator is buried with InP can be found, for example, in theJapanese Patent Application Laid-Open No. 7-193210. However, in thisexample, since the optical waveguide is broken between the DFB laser andthe EA modulator, the light suffers radiation loss at the gap. Themagnitude of the radiation loss is shown in FIG. 20.

[0051]FIG. 20 is a diagram showing a relation between the magnitude ofthe radiation loss and the distance between the optical modulator andthe laser part. The horizontal axis denotes the distance between theoptical modulator and the laser part; the vertical axis denotes theradiation loss in the device. In FIG. 20, the case where InP existsbetween the modulator and the laser part is marked with “buried withInP” and the case of the present invention is marked with “presentinvention.” In the case of “buried with InP,” it is shown that theradiation loss increases exponentially with an increase in distancebetween the modulator and the laser. When the EA modulator and the laserpart are separated by at least a few tens of μm, there occurs aradiation loss equal to at least approximately 1 dB, and the loss maybecome a large obstacle to an increase of the optical output from thedevice.

[0052] Contrary to this, with a structure such that an area where thecrystal defect was removed is buried with a bulk crystal whoserefractive index is preferable for guiding light, as is the case of thepresent invention, little optical radiation loss is generated betweenthe laser part and the modulator. A schematic cross section of such astructure is shown in FIG. 21. FIG. 21 details a DFB laser part 501, tanoptical modulator part 502, and an area of the optical waveguide of thebulk crystal formed by new crystal growth 503. As the laser part 501, afirst optical confinement layer 514, an active layer 515, a secondoptical confinement layer 516, and a grating 517 are preferably formedon a semiconductor substrate 500. For the optical modulator part 502, afirst optical confinement layer 511, an optical waveguide part 512, anda second optical confinement layer 513 are preferably formed.Thereafter, a second cladding layer 518 is formed over these two parts501, 502.

[0053]FIG. 22 is a cross section showing an example where an area of thecrystal defect that existed between the optical modulator 502 and thelaser part 501 is buried with a second cladding layer 518 by crystalgrowth. In this example, in a similar manner to the above-mentionedexample of FIG. 21, the laser part includes a first light confinementlayer 514, an active layer area 515, a second optical confinement layer516, and a grating 517 are formed on the semiconductor substrate 500.The optical modulator includes a first optical confinement layer 511, anoptical waveguide part 512, and a second optical confinement layer 513formed on the substrate 500. Further, a second cladding layer 518 isagain formed over these parts 501, 502. It should be noted in thisexample that a gap between the optical modulator 502 and the laser part501 is buried with the second cladding layer 518.

[0054] Here, FIG. 21 and FIG. 22 are schematic, conceptual diagrams, andthere may be many structural variants that can be carried out inmanufacturing actual devices according to these diagrams.

[0055] According to the present invention, the loss may be reduced by atleast an order of magnitude as compared to the structure illustrated inFIG. 22 where the gap between the modulator and the laser is buried withInP, and a structure suitable to increase the optical output of a devicemay be provided. This improvement may be more fully understood withrespect to the comparison of characteristics shown in FIG. 20 anddescribed above.

[0056] Therefore, it is preferred in the present invention to establishan optical connection between the optical modulator and the laser partwith the optical waveguide of a bulk crystal. If the optical modulatorand the laser part, which are placed with a gap, are connected using thesame material as the cladding layer, one or more of the above desiredeffects may not be obtainable.

[0057] The present invention may be effective for semiconductoroptoelectronic waveguiding devices, especially for semiconductoroptoelectronic waveguiding devices that use compound semiconductormaterials. Typical examples of semiconductor materials to be used forsaid semiconductor optoelectronic waveguiding devices include InGaAlAs,InGaAsP, and similar compounds. The MQW structure may be composed of oneof these enumerated materials. In this case, one material selected froma group consisting of InGaAsP, InGaAlAs, and InAlAs is preferable forthe bulk part for connecting these MQW structures. Generally speaking,InP is used as the substrate for various device applications.

[0058] Further, this invention may be effective for optoelectronicwaveguiding devices that use compound semiconductor materials containingN. In other words, the optoelectronic waveguiding device may use anInGaNAs system compound semiconductor material as a material for the MQWstructure formed on a GaAs substrate.

[0059] First Exemplary Embodiment

[0060] A first exemplary embodiment of the present invention as appliedto a semiconductor laser device into which a 1.55 μm band EA modulatoris integrated (“EA modulator integrated semiconductor laser device”)will now be described. The feedback of light in the present laser may beachieved by distributed feedback (DFB).

[0061]FIG. 1 is a perspective view of a first exemplary embodimentaccording to the present invention, and FIG. 2 is a cross section of theoptical waveguide part taken along a plane parallel to the travellingdirection of light therein. FIG. 3 is a cross section of the devicetaken along a plane intersecting the travelling direction of lighttherein, and FIG. 4 illustrates cross sections of the device duringvarious stages of the manufacturing process.

[0062] As shown in FIG. 1, the device is comprised of two deviceelements, a laser part 26 and a modulator 27. A laser electrode 23 and amodulator electrode 24 were formed independently. Between the laser part26 and the modulator 27, a groove 21 is preferably formed forelectrically isolating both device elements 26, 27. An optical waveguidepart 31 of the device is formed as a stripe shape with a buried hetero(BH) structure as is generally known in the art. In this example, thesides of the stripe optical waveguide in the buried hetero structure arepreferably buried with an Fe-doped high-resistance InP 32 (see FIG. 3).

[0063] The cross section of a layered structure in the presentembodiment is shown in FIG. 2. In order to optimize the devicecharacteristics, each of the laser part 26 and the EA modulator part 27is of an optimal structure independently. Therefore, each element has adifferent layered structure. However, the substrate is preferably commonto both device elements and is a compound semiconductor substrate madeof, for example, n-type InP

[0064] The laser part 26 preferably comprises an n-type InGaAsP opticalconfinement layer 2, a strained MQW layer 3, and a p-type InGaAsPoptical confinement layer 4. The MQW layer, that will act as the activelayer region, may consist of seven pairs (cycles) of a 6 nm thick welllayer and a 10 nm thick barrier layer which are deposited sequentially,with the intention of achieving sufficient characteristics as a laser. Agrating layer 5 made of an InGaAsP material exists on top of theselayers. The active layer region and a structure of the grating layer maybe formed so that the emission wavelength of the DFB laser at roomtemperature (25 degree C.) is approximately 1550 nm.

[0065] The optical confinement layers provided to sandwich the MQW layerare layers for enhancing the optical confinement of the above-mentionedMQW layer. The optical waveguiding function is effected by sandwichingthe core region with cladding layers whose refractive indexes are lowerthan that of the core region. That is, the optical waveguiding functionis achieved by a layered structure of cladding layer/MQW layer/claddinglayer.

[0066] More concretely, the optical confinement layers sandwiching theMQW layer are provided to further enhance the optical confinement in theMQW layer. Therefore, the refractive indexes of the cladding layers areset to be lower than the refractive indexes of the above-mentionedoptical confinement layers. Thus, the optical waveguiding function ispreferably enhanced by the layered structure of the claddinglayer/optical confinement layer/MQW layer/optical confinementlayer/cladding layer. A form may also be adopted in which the claddinglayer of the substrate side is the substrate itself. It may also bepossible to provide the cladding layer of the substrate sideindependently on the semiconductor substrate.

[0067] The polarity of the grating layer 5 may be either n type or ptype. In the case of p type polarity, the DFB laser preferably becomes arefractive index coupling type where only the refractive index variesperiodically in the propagation direction of light. Alternatively, then-type grating polarity gives rise to a gain coupling type DFB laser.The reason being that the grating functions as a periodically varyingcurrent-blocking layer, as is well known in the art. Therefore, not onlythe refractive index but also the gain in the active layer suffersperiodic changes. In the present embodiment, a grating that wasuniformly formed over the whole area of the DFB laser is described.However, there may also be provided a so-called “phase-shifted grating”where a period (phase) of the grating is shifted in a partial areathereof with respect to the remainder.

[0068] On the other hand, the EA modulator area 27 is preferablycomprised of an n-type InGaAsP optical confinement layer 11, an undopedoptical absorption layer 12, and an undoped InGaAsP optical confinementlayer 13. For improved performance of the EA modulator, the opticalabsorption layer 12 may be formed of an InGaAsP system strained multiplequantum well (strained MQW) structure. The thickness of a quantum wellis preferably set at approximately 7 nm, and the thickness of a barrierlayer is set at approximately 5 nm. These layers may be deposited for 10cycles. T barrier layer in the modulator part is preferably thinner thanits counterpart in the laser part because the drift of the carriers isfacilitated in the absorption layer to improve the modulationcharacteristics.

[0069] To remove the crystal defect that develops in the connection partbetween the laser and the modulator, an InGaAsP optical waveguide layermay be formed in this area by butt-joint formation. An exemplary methodof forming the waveguides of the modulator part and the butt-jointwaveguide layer will now be described with reference to FIG. 4.

[0070] initially, to form the laser structure, an n-type InGaAsP opticalconfinement layer 2, a strained MQW layer 3, and a p-type InGaAsPoptical confinement layer 4 are preferably deposited on an n-type InPsubstrate 1.

[0071] On top of these layers, a layered structure including the gratinglayer 5 made of an InGaAsP system material is thereafter formed (FIG.4A).

[0072] Silicon nitride (“SiN”) is coated on the semiconductor waferhaving the above layered structure and is shaped to be a protection mask51 on the laser part. Using this SiN mask 51 that covers the laser partarea, a grating layer 5 and the active layer region in the other areaare etched away, as shown in FIG. 4B. The etching proceeds through then-type InGaAsP layer 2 and is selectively stopped at the n-type InPsubstrate 1. The etching process may be selected from: a dry etchingprocess such as reactive ion etching (RIE); a selective wet etchingprocess using a solution whose main constituent is phosphoric acid orsulfuric acid; and a combination of both of these processes.

[0073] On the n-type InP substrate 1 that was exposed by theabove-mentioned selective etching, an absorption layer area of the EAmodulator is formed by a first butt-joint process (FIG. 4C). Theabsorption layer area is preferably formed by sequentially depositing ann-type InGaAsP optical confinement layer 11, a MQW optical absorptionlayer 12, a p-type optical confinement layer 13, and a p-type InPcladding layer 15.

[0074] The strained-MQW layer 12 may be comprised of ten cycles of a 7nm thick quantum well layer and a 5 nm thick barrier layer. Theabsorption edge thereof is designed to be approximately 1490 nm. Toachieve improved modulation characteristics, crystal compositions of thequantum well and of the barrier well are preferably chosen so that theformer is given with compression strain whereas the latter is given withtensile strain.

[0075] During this first butt-joint process, when the MQW layer of theEA part is formed, a crystal defect 17 with poor crystallinity is alsoformed in the vicinity of the protection mask 51 in the laser part. Inthe crystal defect area 17, crystallinity is bad and the absorption edgeof the MQW has shifted to the longer wavelength side.

[0076]FIG. 5 shows an example of a shift amount of the absorption edgeof the EA part when the EA part is grown with the butt-joint method. Thehorizontal axis denotes the distance from the edge of the protectionmask in the laser part, and the vertical axis denotes the shift amountof the absorption edge of the EA modulator part. In this example, theshift of the absorption edge is caused in the area adjacent to theprotection mask in the laser part within the range of at leastapproximately 100 μm from a mask edge, and the wavelength shift of theabsorption edge is larger as the distance from the protection mask inthe laser part decreases. At the closest proximity to the mask, theabsorption edge has moved to the longer wavelength side by as much asabout 30 nm.

[0077] In order to remove this crystal defect 17, a mask 52 that has anopening only in the vicinity of the crystal defect (FIG. 4D) is formed,and the crystal defect is removed for a length of about 50 μm. Also inthis process, in a similar manner to the first butt-joint process, theetching is selectively stopped at the surface of the n-type InPsubstrate 1. Subsequent to this etching, an optical waveguide layer 14preferably made of undoped InGaAsP and an undoped InP layer 18 aredeposited thereon (FIG. 4E).

[0078] After the optical waveguide structure is formed according to theabove-mentioned procedure, the p-type InP cladding layer 15 and a p-typeInGaAs layer 16 may be formed. An exemplary process for growing thesecrystals uses a metal-organic chemical vapor deposition (MOCVD) method.Further, the p-type InGaAs layer 16 may be formed to obtain an ohmiccontact when the electrode is to be formed. An exemplary process forgrowing the above-mentioned crystal uses a metal organic vapor phaseepitaxy (MOVPE) method. It should be noted in the cross section of FIG.3 that numerals for the p-type InP cladding layer 15 and the p-typeInGaAs layer 16 on the layer 13 are omitted.

[0079] Following the above-mentioned crystal growth process, a buriedhetero (BH) structure is preferably formed by a process of forming amesa stripe through a normal dry etching process and a further processof regrowing a burying layer by the MOVPE method. Here, the BH structureis a structure where both sides of an optical waveguide, as viewed fromthe travelling direction of light, are buried with a material capable ofconfining the light. The confining material normally is characterized bya high resistance. For the burying layer in this example, ahigh-resistance InP 32 was used and Fe was doped therein to increase itsresistivity.

[0080]FIG. 3 is a cross section of the BH laser part taken along a planeintersecting the travelling direction of light therein. FIG. 3 mayfacilitate a better understanding of the BH structure. Following thisprocess, the wafer surface is insulated with silicon oxide (“SiO₂”) 22and the p-side electrode 24 and an nside electrode 25 are then formed.Moreover, on a front facet and on a rear facet of the device, alow-reflection coating 40 and a high-reflection coating 41 are formed,respectively (see e.g., FIG. 2).

[0081] To aid in the understanding of the effect on a device accordingto the present invention, a distribution of optical output efficiencyper unit current (slope efficiency) obtained by evaluating the opticaloutput versus current characteristics of exemplary devices is shown inFIG. 6. The horizontal axis of FIG. 6 denotes the slope efficiency, andthe vertical axis denotes the cumulative distribution for the measuredsamples. FIG. 6 also shows the cumulative distribution of slopeefficiency for devices that the present invention was not applied to(conventional device) and hence have crystal defects at the connectionparts between the laser part and the modulator.

[0082] In FIG. 6, a group of solid squares designates samples accordingto the present invention, and a group of open circles designates samplesof conventional devices. About 95 percent of the conventional devicesare distributed below a slope efficiency of approximately 0.15 W/A. Bycontrast, less than about 25% of the devices according to the presentinvention have a slope efficiency of 0.15 W/A or less. Further, focusingon the center value of the distribution corresponding to 50 percent ofthe probability distribution, the devices according to the presentinvention improved the center value (almost equivalent to the averagevalue) of the slope efficiency by a factor of about 1.3 (2 dB) over theconventional devices. One reason for this changes is that because thedevices of the present invention have no crystal defect in the opticalwaveguides, the absorption loss in the waveguide is reduced to anextremely small value as compared to the conventional butt-joint method.

[0083] Second Exemplary Embodiment

[0084]FIG. 7 though FIG. 9 show devices according to a second exemplaryembodiment of the present invention. FIG. 7 is a perspective view of adevice of the second embodiment, and FIG. 8 is a cross section of thewaveguide part of the device taken along a plane parallel to thetravelling direction of light therein. FIG. 9 is a cross section of thedevice taken along a plane intersecting the travelling direction oflight therein, and FIG. 10 shows cross sections illustrating anexemplary manufacturing process of the device.

[0085] This embodiment is preferably characterized in that the EAmodulator part 27 is comprised of an InGaAlAs system MQW. The laser part26 is formed with an InGaAsP system MQW in a similar manner to the firstexemplary embodiment. There is a groove 21 that separates these MQWstructures. As best seen in the perspective view of FIG. 7, the presentexemplary embodiment has a basic configuration similar to that describedwith respect to the first embodiment. A detailed explanation for thesimilar features in the figures is omitted.

[0086] It is known that an EA modulator that uses an InGaAlAs systemmaterial has improved modulation efficiency and may achieve a largerextinction ratio than one that uses an InGaAsP system MQW. Theextinction ratio is a parameter which shows the ratio of the lightoutputs for ON and for OFF of the optical signal. In general, larger theextinction ratio are preferred for the transmission of light signals.Therefore, for an optical transmission system that requires a largerextinction ratio, the laser having the EA modulator made of the InGaAlAssystem MQW integral therewith, as described in this embodiment, may besuitable. Hereafter, an exemplary manufacturing process for the laserwill be described with reference to FIG. 10.

[0087] The growth temperature of the InGaAlAs system material to be usedfor the EA modulator part in this embodiment is higher than the growthtemperature of the InGaAsP system material to be used for the laserpart. Therefore, if the MQW of the laser part is formed first, that MQWwill be exposed to an elevated temperature above the growth temperatureof the laser. When the laser part undergoes such a thermal hysteresis,minute defects in the crystals that constitute the MQW of the laser partmove easily to easily effectuate a degradation of the crystals, andthese defects cause a deterioration of the device characteristics. Tocircumvent this potential problem, the InGaAlAs-system MQW that requiresthe high temperature is preferably grown first in the present embodimentof the invention.

[0088] The optical absorption layer preferably comprises: an n-typeInGaAlAs optical confinement layer 112; an MQW layer 113; and an undopedInGaAlAs optical confinement layer 114 formed on an n-type InP substrate111. The MQW layer 113 may be manufactured by depositing a quantum welllayer (thickness approximately 7 nm) and a barrier layer (thicknessapproximately 5 nm), both of which consist of the InGaAlAs systemmaterial, for ten cycles. On this optical absorption layer, a claddinglayer of a p-type InP 115 is grown to a thickness of approximately 0.2μm.

[0089] The semiconductor wafer that has this layered structure is cladwith SiN, which is formed to a mask 116 that serves to protect themodulator part (FIG. 10A). Using this SiN mask 116, the p-InP claddinglayer and the optical absorption layer are etched away (FIG. 10B). Theetching is allowed to proceed through the n-type InGaAlAs 112 and isselectively stopped at the n-type InP substrate 111. The etching may beeither of: dry etching, for example, reactive ion etching (RIE);selective wet etching that uses a solution whose main constituent isphosphoric acid or sulfuric acid; or a combination of both of theseprocesses.

[0090] On the n-InP substrate 111 that has been exposed by the etching,an InGaAsP system MQW structure that will act as a laser is formed by afirst butt-joint process. The MQW structure is manufactured, in asimilar manner to the first embodiment, by depositing an n-type InGaAsPoptical confinement layer 121, an undoped active layer 122, an undopedInGaAsP optical confinement layer 123, and additional layers thereon.Then, a grating layer 124 made of an InGaAsP system material and ap-type InP cladding layer 125 are preferably formed (FIG. 10C).

[0091] Next, a crystal defect 126 with poor crystallinity that wasformed at the connection part between the modulator and the laser isremoved using a SiN mask 130 (FIG. 10D). In the removal process, in asimilar manner to the first butt-joint process, dry etching such as thereactive ion etching (RIE) and selective wet etching using a solutionwhose main constituent is phosphoric acid or sulfuric acid arepreferably used together. It should be noted that the etching isselectively stopped at the n-type InP substrate 111. After the InPsubstrate 111 is exposed, an undoped bulk InGaAsP waveguide layer 131and an undoped InP 132 are deposited thereon as a second butt-jointprocess (FIG. 10E).

[0092] After the two butt-joint processes described above, a gratingstructure 141 is formed in the laser area by a conventional method (FIG.10F) on which a p-type InP cladding layer 151 and a p-type InGaAs layer152 are deposited (FIG. 10G).

[0093] Thereafter, a laser structure is constructed through the sameprocess as the first embodiment. As details are the same as describedwith respect to the first embodiment, further explanation is omitted.FIG. 9 is a cross section of the device taken along a plane intersectingthe travelling direction of light therein. In FIG. 9, reference numeralsfor the p-type InP cladding layer 151 and for the p-type InGaAs layer152 on the layer 114 are omitted.

[0094] Exemplary optoelectronic waveguiding devices according to thisembodiment are evaluated for optical output versus currentcharacteristics at an operating temperature of 25 degree C. The resultsof the tests show that an average threshold current was approximately 8mA, and the average slope efficiency was 0.17 W/A. An example of thecumulative distribution of slope efficiency was shown in FIG. 6. FromFIG. 6, it can be understood that the present invention preferablyimproves the slope efficiency as compared to conventional devices.Further, exemplary optoelectronic waveguiding devices according to thisembodiment are tested in an optical transmission experiment using asingle mode optical fiber of a length of 40-80 km, and sufficienttransmission characteristics are achieved.

[0095] Moreover, in an acceleration test for the reliability, anestimated life exceeding 200,000 hours is successfully secured.

[0096] Third Exemplary Embodiment

[0097] As a third exemplary embodiment of the present invention, asapplied to a 1.55 μm band EA modulator integrated distributed feedbacksemiconductor laser, examples in which the bulk crystal waveguideprovided at the connection part between the EA modulator and the laserhas a mode-conversion function are shown in FIG. 11 and FIG. 13. Theseaddress the potential problem of mode mismatching between the waveguidesin cases where the EA modulator part and the laser part have differentwaveguide layer thicknesses, and therefore, the guided mode size oflight in the respective areas differ from each other.

[0098] The exemplary embodiment illustrated in FIG. 11 is an example inwhich the length of the modulator is shortened with the intention ofimproving the operating frequency band thereof. That is, by shorteningthe modulator length of the modulator, the electric capacitance can bereduced, and the modulation bandwidth of the device can be improved. Inthis example, the thicknesses of several layers that constitute theoptical modulator are set to be larger than those of the correspondinglayers of the laser part. FIG. 11 is a cross section of the device takenalong a plane parallel to the travelling direction of light therein, andFIG. 12 is a top view showing locations and widths of patterns of theprotection mask that are used to manufacture respective parts in thisexample.

[0099] In an example shown in FIG. 13, the thicknesses of layers thatconstitute the optical modulator are set to be smaller than those of thecorresponding layers of the laser part. FIG. 13 is a cross section ofthe device taken along a plane parallel to the travelling direction oflight therein, and FIG. 14 is a top view showing locations and widths ofpatterns of a protection mask that are used to manufacture respectiveparts of this example.

[0100] First, the example of FIG. 11 will be described. In thisstructure, to achieve a sufficient extinction ratio with a shortmodulator, the number of the quantum well layers of the EA modulationpart was increased, and the optical confinement coefficient in theabsorption layer was made larger. Accordingly, the thickness of awaveguide 204 in an area near the EA modulator part is larger than thatin an area near the laser part. Also in this embodiment, the modulatorpart is preferably comprised of the MQW structure of an InGaAlAs systemmaterial to realize desired modulation characteristics of the modulator.An exemplary manufacturing process for this embodiment will now bedescribed.

[0101] According to the same procedure that is executed in the secondembodiment, an optical absorption layer comprises an n-type InGaAlAsoptical confinement layer 201, an MQW layer 202, and an undoped InGaAlAsoptical confinement layer 203 formed on an n-type InP substrate 111. Thequantum well layer acting as a light absorbing layer is repeatedlydeposited to approximately 14 cycles which is larger than that of thesecond embodiment. Accordingly, the absorption layer is thicker thanthat of the second embodiment by approximately 50 nm.

[0102] On this optical absorption layer, a cladding layer of p-type InPis grown to a thickness of about 0.2 μm. On the semiconductor waferhaving this layered structure, a SiN mask for protecting the modulatorpart is formed using the same technique as the second embodiment. Usingthis SiN mask, the p-InP cladding layer and the optical absorption layerare etched away. The etching is allowed to proceed through the n-typeInGaAlAs 201 and is selectively stopped at the n-type InP substrate 111.The etching may be either of: dry etching, for example, reactive ionetching (RIE); selective wet etching that uses a solution whose mainconstituent is phosphoric acid or sulfuric acid; or a combination ofthese processes.

[0103] Using the above procedure, the surface of the n-type InPsubstrate 111 is exposed, and an active layer region of the laser isformed on this surface. The structure of the active layer region is thesame as in the first and second embodiments. The n-type InGaAsP opticalconfinement layer 2, the active layer 3, the undoped InGaAsP opticalconfinement layer 4 are formed, On these layers, the grating layer 5made of an InGaAsP system material and a p-type InP cladding layer areformed.

[0104] Subsequently, in a similar manner to the second embodiment, thecrystal defect that was formed at the connection part between theoptical modulator and the laser part is replaced with a bulk InGaAsPwaveguide 204 by a second butt-joint process. In this embodiment, themask pattern to be used in this second butt-joint process is such thatthe width of a mask stripe pattern on the optical modulator side is setto be larger than that on the laser side. By the selective use of stripepattern widths in the process of bulk crystal growth of a compoundsemiconductor material, the bulk semiconductor layers can be tailored tobe thicker near the modulator than near the laser. Thus, as illustratedin FIG. 11, the waveguide made of the bulk compound semiconductor can bemanufactured with a tapered structure.

[0105] Also in the process of removing the MQW structure using theabove-mentioned mask, in a similar manner to the first butt-jointprocess, it is recommended that the dry etching such as the reactive ionetching (RIE) and the selective wet etching that uses a solution whosemain constituent is phosphoric acid or sulfuric acid are used together.The etching is selectively stopped at the n-type InP substrate 111.After the n-InP substrate 111 is exposed, the undoped bulk InGaAsPwaveguide layer 112 and an undoped InP are preferably deposited as asecond butt-joint process.

[0106] As described above, in the second butt-joint growth process, ifthe width of a mask 211 on the EA modulator side is set to be largerthan the width of a mask 212 for protecting the laser part, thewell-known effect of the selective area growth occurs. That is, sincethe quantity of the growth species supplied becomes large near the EAmodulator side, the crystal that is grown there in the second butt-jointprocess becomes thicker than that near the laser side. Accordingly, theInGaAsP waveguide layer 204 tends to be thicker closer to the modulatorside than at a point near the laser side.

[0107] As a result, the thickness of the waveguide can be varied in ataper manner so that the thickness of the waveguide near the laser sideequals the thickness of the waveguide of the laser and the waveguidenear the EA modulator side equals the thickness of the absorption layer.Thus, any discontinuity of the waveguide thickness may be eliminatedboth at the connection part between the laser part and the InGaAsP bulkcrystal waveguide and at the connection part between the opticalmodulator and the InGaAsP bulk crystal waveguide. Therefore, thepropagation loss of light between the waveguides is decreased.

[0108] After the second butt-joint process is completed according to theabove-mentioned procedure, a grating structure is formed in the laserarea by a conventional technique, and a p-type InP cladding layer 205and a p-type InGaAs layer 206 are deposited thereon. Subsequently, thelaser structure is formed through the same process that is describedwith respect to the first embodiment. Previously described details areagain omitted.

[0109] Moreover, the above-mentioned description is for the case wherethe waveguide thickness of the EA modulator portion is larger than thatof the laser part, but the present invention can be applied to aconverse case (shown in FIG. 13) where the waveguide thickness of the EAmodulator part is smaller than that of the laser part. In that case, anoptical waveguide 251 of a bulk crystal that is provided between thelaser part and the modulator is such that its thickness at theconnection part to the laser part is thick and becomes thinner nearer tothe modulator.

[0110] In the case where the optical waveguide 251 with such a tapershape is formed, a mask pattern as shown in FIG. 14 may be used in thesecond butt-joint process. That is, the width of a SiN mask 261 (stripepattern) for protecting the laser side is to be larger than the width ofa SiN mask 262 (stripe pattern) for protecting the EA modulator part.The reason that the taper waveguide is formed with such a mask patternis the same as description of FIG. 12.

[0111] Note that in the embodiments shown in FIG. 11 and in FIG. 13, thedevice manufacturing process is basically the same as the firstembodiment excluding alteration of the mask pattern.

[0112] Fourth Exemplary Embodiment

[0113]FIG. 15 and FIG. 16 are examples of the present invention asapplied to side-by-side arrangements of the device elements described inthe first and/or second embodiments to compose an array ofoptoelectronic device elements. FIG. 15 is a top view of the deviceaccording to this embodiment, and FIG. 16 is a cross section of thedevice taken along a plane parallel to the travelling direction of lighttherein. Note that each figure shows only characteristic portions of thedevice, and portions common to the device according to the presentinvention (such as the electrodes) are omitted for clarity.

[0114] A device with a plurality of modulator-integrated lasers arrangedside by side, as in this example, provides a plurality of opticalsignals to be used with a plurality of transmission channels. Thereforethe present invention can provide light sources suitable to a furtherhigh-capacity optical transmission system such that the transmissioncapacity is proportional to a product of the transmission speed and thenumber of channels.

[0115]FIG. 15 shows a top view of the device with four pieces of DFBlasers each acting as a light emitting part arranged side by side, froma first channel laser 301 to a fourth channel laser 304 mounted on itssurface. Further, four modulators, from a first channel modulator 311 toa fourth channel modulator 314, are also formed in such a way that eachmodulator corresponds to one of the DFB lasers. FIG. 16 shows across-sectional structure of the optical waveguide of the side-by-sidearranged channel device. Each of the four channels has a laser area 321and a modulator area 322, and a bulk crystal waveguide 323 is formedbetween these two areas. The formation process for these waveguidestructures is preferably the same as that described in the firstembodiment.

[0116] In this case, there can be expected a decrease in powerconsumption of the device and of an optical module with the deviceincorporated therein. This expectation stems from the fact (alreadydiscussed in the first exemplary embodiment) that the application of thepresent invention increases the optical output of each channel by about2 dB. This increase in output means that a lower amount of electricpower is necessary to obtain the same quantity of optical output as aconventional device. In the optoelectronic waveguiding device in which aplurality of device elements are arranged side by side as shown in FIG.15, power consumption of the entire device is a product of powerconsumption of each device element and the number of channels.Therefore, the decrease in power consumption for the entire device is aproduct of the decrease in power consumption for each channel and thenumber of channels. That is, the device has a greater decrease in powerconsumption then a single channel device.

[0117] Fifth Exemplary Embodiment

[0118] Moreover, an embodiment shown in FIG. 17 is an example of thedevice such that light emitted from one of a plurality of lasersarranged side by side is multiplexed to a single optical waveguide by amultiplexer and then modulated by a single optical modulator. FIG. 17includes a laser part 451, an optical modulator part 452, and an opticalmultiplexer part 411, all of which are connected to one another withoptical waveguides. Four lasers for four channels, from a first channellaser 401 to a fourth channel laser 404, are arranged side by side.Light generated by any of the four channel lasers is preferably made toenter a single optical modulator 412 through the optical multiplexer411, and is modulated thereby.

[0119] With a device as depicted in FIG. 17, it is possible that adesired wavelength (i.e., one of four wavelengths) is obtained from asingle device. In this embodiment, emission wavelengths of the fourchannel lasers, from the first channel laser 401 to the fourth channellaser 404, are preferably set at different wavelengths with a spacing ofapproximately 2 nm. Specifically, a device with lasers at frequencies of1550 nm, 1552 nm, 1554 nm, and 1556 nm, respectively was formed.Adoption of such a side-by-side structure makes it possible to obtainlight having a desired wavelength by injecting current to a channelhaving the desired wavelength according to need. Note that in thisembodiment the number of channels and wavelength separation between thechannels are set at 4 and 2 nm, respectively, but by setting the numberof channels and wavelength separation according to need, the number ofselectable wavelengths and a wavelength region can be set in an almostunlimited variety of desired values.

[0120]FIG. 18 is an irregular section of the optoelectronic waveguidingdevice taken along a non-straight line that passes each opticalwaveguide layers. In order that the laser part 451 and the EA modulatorpart 452 possess their optimal characteristics, respectively, bothdevice elements preferably have MQW structures. As details of theirlayered structures are the same as in the first embodiment, furtherexplanation is omitted.

[0121] Between the lasers and the EA modulator, namely at a positionwhere the multiplexer is provided, an optical waveguide layer 453 isformed of a bulk crystal having a refractive index distributiondesirable as the optical waveguide according to the present invention.In the case where a multiplexer is provided, as in the presentembodiment, because the length of the whole device becomes longer, itmay be important to limit the optical absorption per unit length in thepath. If the light is absorbed in the optical waveguide, optical outputemitted from the device becomes small. As a result, after the device isbuilt into an optical transmission apparatus, there may occur a failurethat results in deterioration of S/N characteristics of the transmissionapparatus. The optical multiplexer itself may be of a conventionalstructure, and many types of device elements may achieve a sufficienteffect.

[0122] To reduce optical loss in an optical waveguide, the structureaccording to the present invention where a bulk crystal waveguide isprovided between the laser part and the EA modulator is preferablysuitable. As already described, since the bulk crystal waveguide has anabsorption edge that is far away from the guided wave wavelength ascompared to the MQW waveguide, it is possible to control the waveguideabsorption per unit length to be low value. Further, although in thepresent exemplary embodiment the number of channels is set at 4, if thepresent invention is applied to a device whose number of channels isother than four, the effect of reducing the loss when the light passesthrough the multiplexer is the same.

[0123] A conceptual top view of a mounting form including anoptoelectronic waveguiding device according to this embodiment is shownmounted on an optical module in FIG. 19. An optoelectronic waveguidingdevice 603 is placed on a mount board 602 together with an optical lens604, and the prepared mount board 602 is then placed in a package 605and a fiber 601 is connected thereto. Current feed to the device may beaccomplished from an electrode pad through a wire in a similar manner toconventional techniques. In FIG. 19, there is shown a feed pad 611 forEA modulator; an n-electrode pad 612 for semiconductor laser;p-electrode feed pads 621, 622, 623, and 624 for DFB laser; and a feedwire 631.

[0124] As described in detail in the foregoing, according to the presentinvention, a modulator-integrated laser that emits large optical outputand enables high-speed optical communication can preferably be provided.With the use of the optical module and/or optical transmission apparatusinto which the device according to the present invention was built, ahigh-speed optical transmission system capable of operating at low powerconsumption can be provided.

[0125] Some additional exemplary forms according to the presentinvention will now be enumerated and briefly described.

[0126] (1) An optoelectronic waveguiding device in which a layeredstructure consisting of an optical waveguide layer having a refractiveindex desirable for light guiding and cladding layers each of which ismade of a material having a refractive index lower than that of theoptical waveguide layer that exist above or beneath the opticalwaveguide is formed on a semiconductor substrate. The device ischaracterized in that (a) a plurality of optical waveguide layers eachhaving the MQW structure exist in a direction of a light propagationaxis of the optical waveguide layer each have layer structures orconstituent materials that are mutually different, and (b) an opticalwaveguide made of a semiconductor bulk crystal that bears a smallquantum effect and has a refractive index higher than those of thecladding layers exists at the connection part between these MQWstructure waveguides.

[0127] (2) An optoelectronic waveguiding device formed by depositing afirst MQW structure on a semiconductor substrate, removing a part of thefirst MQW layer and subsequently forming a second MQW structure in thatarea. Further, an optical waveguide layer made of a semiconductor bulkcrystal material that bears a small quantum effect and has a refractiveindex higher than those of the cladding layers is deposited thereon.

[0128] (3) An optoelectronic waveguiding device, characterized in that aplurality of optoelectronic waveguiding device elements each having adifferent function are formed in a direction of the light propagationaxis on the same semiconductor substrate. In these optoelectronicwaveguiding device elements, cladding layers are each made of a materialhaving a refractive index lower than that of the optical waveguide layerbeing deposited above and beneath the optical waveguide layer, and someor all of the optoelectronic waveguiding device elements have the MQWstructures. At the connection part between the optoelectronicwaveguiding device elements, an optical waveguide exists that is formedby depositing a semiconductor bulk crystal bearing an extremely smallquantum effect.

[0129] (4) An optoelectronic waveguiding device that has at least asemiconductor laser part and an optical modulator in a portion thereof,characterized in that both the optical modulator and the semiconductorlaser part have the MQW structures, layer structures or constituentmaterials of these MQW structures are mutually different, and an opticalwaveguide made of a bulk crystal exists at the connection part betweenthe optical modulator and the semiconductor laser part.

[0130] (5) An optoelectronic waveguiding device, characterized in thatoptoelectronic waveguiding device elements each of which is oneoptoelectronic waveguiding device element selected from a groupconsisting of at least a semiconductor laser, an optical modulator, anda semiconductor optical amplifier are formed on the same semiconductorsubstrate. These optoelectronic waveguiding device elements are arrangedin a direction of the light propagation axis with at least the opticalmodulator and the semiconductor laser part having the MQW structures.Layer structures or constituent materials of these MQW structures aremutually different. An optical waveguide that is formed by depositing asemiconductor bulk crystal bearing an extremely small quantum effectexists at the connection parts between two of the optical modulator, thesemiconductor laser, and the semiconductor amplifier.

[0131] (6) An optoelectronic waveguiding device, characterized in that(a) a plurality of device element structures each comprising at least asemiconductor laser and an optical modulator are formed side by side ina direction parallel to the light propagation axis on the samesemiconductor substrate, each device element structure capable ofemitting light of the same or a mutually different wavelength. Both theoptical modulator and the semiconductor part having the MQW structures,respectively, and layer structures or constituent materials of these MQWstructures being mutually different. An optical waveguide made of a bulkcrystal may exist at the connection part between the modulator and thesemiconductor laser part.

[0132] (7) An optical module that has at least a semiconductoroptoelectronic waveguiding device and an optical fiber as constituentsin a portion thereof, characterized in that said semiconductoroptoelectronic waveguiding device is a semiconductor optoelectronicwaveguiding device such that a plurality of device element structureseach comprising at least a semiconductor laser and an optical modulatorare formed side by side in a direction parallel to the light propagationaxis. Each device element structure being capable of emitting light ofthe same or a mutually different wavelength.

[0133] The optical modulators and the semiconductor laser parts allhaving the MQW structures, and layer structures or constituent materialsof these MQW structures of each device element structure are mutuallydifferent. An optical waveguide made of a bulk crystal exists at theconnection part between the modulator and the semiconductor laser ofeach device element structure.

[0134] (8) An optoelectronic waveguiding device, characterized in that aplurality of device element structures each having at least asemiconductor laser are formed side by side in a direction parallel tothe light propagation axis on the same semiconductor substrate and atleast an optical multiplexer and an optical modulator are arranged inthe optical waveguide running from the semiconductor lasers up to alight facet of the device. The semiconductor lasers, the opticalmultiplexer, and the optical modulator being configured so that lightemitted from the lasers arranged side by side is multiplexed to oneoptical waveguide and subsequently made to enter the optical modulator.

[0135] (9) An optical module comprising at least an optical fiber and asemiconductor optoelectronic waveguiding device capable of emittingsingle longitudinal mode light in a portion thereof, characterized inthat (a) a plurality of device element structures each having at least asemiconductor laser are formed side by side in a direction parallel tothe light propagation axis on the same semiconductor substrate, eachdevice element structure in a side-by-side arrangement being capable ofemitting light of the same or a mutually different wavelength, (b) atleast an optical multiplexer and an optical modulator are arranged inthe optical waveguides running from the semiconductor lasers up to alight emitting facet of the device, the optical multiplexer and theoptical modulator being configured in such a way that light emitted fromany of the lasers in a side-by-side arrangement is multiplexed by theoptical multiplexer and then made to enter the optical modulator, and atleast modulator part having the MQW structure, and (c) an opticalwaveguide made of a bulk crystal exists at least in a portion of eachwaveguide in the optical multiplexer, hence being capable of emittinglight having one of plural wavelengths to the outside of the module.

[0136] (10) An optoelectronic waveguiding device, characterized in thatthe MQW structure made of an InGaAlAs system material and the MQWstructure made of an InGaAsP system material exist on the lightpropagation axis of the optoelectronic waveguiding device, and anoptical waveguide made of one crystal selected from among InGaAsPsystem, InGaAlAs system, and InAlAs system bulk crystals exist at theconnection part between these two MQW structures.

[0137] (11) An optoelectronic waveguiding device that has at least asemiconductor laser and an optical modulator (EA modulator) in a portionthereof, characterized in that a layered structure that constitutes theEA modulator comprises the MQW structure made of an InGaAlAs systemmaterial, a layered structure that constitutes the semiconductor lasercomprises the MQW structure made of an InGaAsP system material, and anoptical waveguide made of one material selected from among InGaAsPsystem, InGaAlAs system, and InAlAs system bulk crystals exists at theconnection part between the modulator and the semiconductor laser.

[0138] (12) An optoelectronic waveguiding device that has at least asemiconductor laser and an optical modulator (EA modulator) in a portionthereof, characterized in that each of the EA modulator and thesemiconductor laser has the MQW structure made of an InGaAsP systemmaterial, at least one of a thickness of the quantum well, a thicknessof the barrier layer, and the number of cycles of the quantum wellswhich constitute the MQW structure of the modulator being different froma counterpart of the MQW structure of the laser part, and an opticalwaveguide made of one material selected from among InGaAsP system,InGaAlAs system, and InAlAs system bulk crystals exists at theconnection part between the modulator and the semiconductor laser.

[0139] (13) An optoelectronic waveguiding device that has at least asemiconductor laser and an optical modulator (EA modulator) in a portionthereof, characterized in that each of the EA modulator and thesemiconductor laser has a MQW structure made of an InGaAlAs systemmaterial, one of a thickness of the quantum well, a thickness of barrierlayer, and the number of cycles of the quantum well which constitute theMQW structure of the modulator is different from a counterpart of theMQW structure of the laser part, and an optical waveguide made of onematerial selected from among InGaAsP system, InGaAlAs system, and InAlAssystem bulk crystals exists at the connection part between the modulatorand the semiconductor laser part.

[0140] Nothing in the above description or provided examples is meant tolimit the present invention to any specific materials, geometry, ororientation of elements. Many part/orientation substitutions arecontemplated within the scope of the present invention and will beapparent to those skilled in the art. The embodiments described hereinwere presented by way of example only and should not be used to limitthe scope of the invention.

[0141] Although the invention has been described in terms of particularembodiments in an application, one of ordinary skill in the art, inlight of the teachings herein, can generate additional embodiments andmodifications without departing from the spirit of, or exceeding thescope of, the claimed invention. Accordingly, it is understood that thedrawings and the descriptions herein are proffered by way of exampleonly to facilitate comprehension of the invention and should not beconstrued to limit the scope thereof.

What is claimed is:
 1. An optoelectronic waveguiding device, comprising:at least two optoelectronic device elements, each having an opticalwaveguide; and an optical waveguide made of a bulk crystal, wherein eachof said optical waveguides in said at least two optoelectronic deviceelements are connected to each other with said optical waveguide made ofa bulk crystal.
 2. An optoelectronic waveguiding device according toclaim 1, wherein at least two of the optoelectronic device elements arebuilt into a single semiconductor substrate.
 3. An optoelectronicwaveguiding device according to claim 2, wherein at least a first ofsaid at least two optoelectronic device elements is a semiconductorlaser part and at least a second of said at least two optoelectronicdevice elements is an optical modulator part.
 4. An optoelectronicwaveguiding device according to claim 3, further comprising: a pluralityof pairs, each pair comprising one of said semiconductor laser parts andone of said optical modulator parts, each of said plurality of pairsformed on a single substrate.
 5. An optoelectronic waveguiding deviceaccording to claim 1, wherein the optical waveguides of said at leasttwo optoelectronic device elements each include a multiple quantum well(MQW) structure existing in a direction of a light propagation axisthereof.
 6. An optoelectronic waveguiding device according to claim 5,wherein said at least two optoelectronic device elements are furthercomprised of: cladding layers each placed above or beneath the opticalwaveguides of said at least two optoelectronic device elements, whereinsaid cladding layers are comprised of a material having a refractiveindex lower than that of the optical waveguides of said at least twooptoelectronic device elements.
 7. An optoelectronic waveguiding deviceaccording to claim 6, wherein said optical waveguide made of a bulkcrystal comprises a semiconductor bulk crystal having a refractive indexhigher than that of said cladding layers.
 8. An optoelectronicwaveguiding device according to claim 5, wherein said at least twooptoelectronic device elements including an MQW structure have mutuallydifferent functions.
 9. An optoelectronic waveguiding device accordingto claim 8, wherein said two optoelectronic waveguiding device elementshaving mutually different functions are a semiconductor laser part andan optical modulator part.
 10. An optoelectronic waveguiding deviceaccording to claim 5, wherein said at least two optoelectronic deviceelements each having an MQW structure include a semiconductor laserpart, an optical modulator part, and a semiconductor optical amplifierpart; wherein at least either the layer structures or constituentmaterials of the MQW structures are mutually different, and furtherwherein said optical waveguide made of a bulk crystal exists atconnection points between said optical modulator part and saidsemiconductor laser part and between said optical modulator part andsaid semiconductor optical amplifier part.
 11. An optoelectronicwaveguiding device according to claim 9, wherein the MQW layer of thelaser part has a different thickness than the MQW layer of the modulatorpart, further wherein said bulk crystal optical waveguide is tapered tointerconnect the different thicknesses.
 12. An optoelectronicwaveguiding device according to claim 5, wherein said at least twooptoelectronic device elements including an MQW structure include aplurality of laser parts and a modulator part, said device furthercomprising: an optical multiplexer including a, MQW structure; whereinsaid optical waveguide made of a bulk crystal is adapted to connect saidplurality of laser parts to said modulator part through saidmultiplexer, further wherein light emitted from one of the semiconductorlaser parts is multiplexed by said optical multiplexer and made to entersaid optical modulator,
 13. An optoelectronic waveguiding deviceaccording to claim 1, wherein said at least two optical waveguides ofthe optoelectronic device elements are a MQW structure made of anInGaAlAs system material and a MQW structure made of an InGaAsP systemmaterial, and said bulk crystal optical waveguide is made of a bulkcrystal that is selected from a group consisting of InGaAsP system,InGaAlAs system, and InAlAs system materials.
 14. An optoelectronicwaveguiding device according to claim 3, wherein a layered structurethat constitutes said optical modulator part comprises a MQW structuremade of an InGaAlAs system material, a layered structure thatconstitutes said semiconductor laser part comprises a MQW structure madeof an InGaAsP system material, and said bulk crystal optical waveguideis made of a bulk crystal that is selected from a group consisting ofInGaAsP system, InGaAlAs system, and InAlAs system materials.
 15. Anoptoelectronic waveguiding device according to claim 3, wherein bothsaid optical modulator part and said semiconductor laser part have anMQW structure made of an InGaAsP system material; wherein one of athickness of the quantum well, a thickness of the barrier layer, and thenumber of cycles of the quantum wells which constitute the MQW structureof said optical modulator part is different from a counterpart of theMQW structure of the semiconductor laser part; and further wherein theoptical waveguide made of a bulk crystal that is selected from a groupconsisting of InGaAsP system, InGaAlAs system, and InAlAs systemmaterials exists at a connection part between said optical modulatorpart and said semiconductor laser part.
 16. An optoelectronicwaveguiding device according to claim 3, wherein both said opticalmodulator part and said semiconductor laser part have an MQW structuremade of an InGaAlAs system material; wherein one of a thickness of thequantum well, a thickness of the barrier layer, and the number of cyclesof the quantum wells which constitute the MQW structure of said opticalmodulator part is different from a counterpart of the MQW structure ofthe semiconductor laser part; and further wherein the optical waveguidemade of a bulk crystal that is selected from a group consisting ofInGaAsP system, InGaAlAs system, and InAlAs system materials exists at aconnection part between said optical modulator part and saidsemiconductor laser part.
 17. An optical module, comprising: an opticalfiber; and a semiconductor optoelectronic waveguiding device comprisinga plurality of device element structures each having at least asemiconductor laser part and an optical modulator part arranged side byside in a direction parallel to a light propagation axis in saidsemiconductor optoelectronic waveguiding device, wherein said deviceelement structures arranged side by side are capable of emitting lighthaving the same wavelength or mutually different wavelengths, both saidoptical modulator part and said semiconductor laser part of each deviceelement structure having MQW structures, wherein layer structures orconstituent materials of these MQW structures of each device elementstructure are mutually different, and further wherein an opticalwaveguide made of a bulk crystal exists at a connection part of saidoptical modulator part and said semiconductor laser part of each deviceelement structure.
 18. An optical module according to claim 17, whereinsaid plurality of device element structures includes a plurality ofsemiconductor laser parts and an optical modulator part, said devicefurther comprising: an optical multiplexer including an opticalwaveguide capable of multiplexing light emitted from one of thesemiconductor laser parts arranged side by side and subsequentlytransmitting the light to said optical modulator part; wherein saidsemiconductor optoelectronic waveguiding device is capable of emittingsingle longitudinal mode light, wherein said semiconductor laser partsand said optical modulator parts being formed on the same semiconductorsubstrate, and further wherein the optical waveguide made of a bulkcrystal exists at least in a portion of the optical waveguide in theoptical multiplexer.
 19. A method for manufacturing an optoelectronicwaveguiding device, comprising the steps of: forming a firstoptoelectronic device element on a semiconductor substrate; applying afirst resist layer to said first optoelectronic device element; etchingsaid first optoelectronic device element; forming a secondoptoelectronic device element in said etched area on said semiconductorsubstrate; applying a second resist layer; etching said second resistlayer to remove a crystal defect formed between said first and secondoptoelectronic elements; and forming a waveguide from a bulk crystal insaid etched crystal defect area, wherein said waveguide from a bulkcrystal optically connects said first and second optoelectronicelements.
 20. The method of claim 19, wherein said first and secondoptoelectronic elements include MQW structures.